Low Oxygen Ocean Water Leads to Mass Extinction?

[Cloral]

I was watching Nova yesterday and they had a piece about a lake in New York that below a certain depth is toxic. The reason, it seems, is because the lake is narrow and deep. Thus, the lake surface cannot absorb oxygen from the atmosphere quickly enough to keep the water well-oxygenated. Thus the deeper lake waters tend to be anaerobic. Under such conditions normal life cannot survive. However, there is a class of bacteria that thrives in this condition. This bacteria produces hydrogen sulfide as a result of its metabolism process. Hydrogen sulfide is the gas that is responsible for the 'rotten eggs' smell, and in large quantities it can be fatal.

The scientist they were following in the piece was studying the lake because he theorizes that such conditions in wide scale led to the Permian Mass-Extinction. Studies of the Permian geologic boundary have shown that the oceans at that time were very poorly oxidized. This would have led to the lake-bottom conditions occurring across the entire ocean. And such a wide-scale release of hydrogen sulfide would have killed a majority of the life not just in the ocean, but on land as well.

How could such conditions arise in the oceans? Two words: global warming. Warmer air temperatures lead to increased stratification of the waters, reducing deep water mixing. Deeper waters are starved of their oxygen, and eventually become anaerobic. The bacteria blooms, and everything else dies.

[Prrkitty]

I watched that very same show this weekend! What they're studying could very well happen again but on a much grander scale... that's scary.

[aces2022]

That's astounding! Gotta love knowing things. Kinda, well really freaking scary! Is there anything known that can kill the bacteria?

[rock_nog]

Um, I don't exactly think that is the best approach. We're killing enough stuff in the oceans as it is, thanks to things like overfishing, fertilizer runoff, and CO2 emissions. And no, I'm not being all lovey-dovey "save the fishes" and all that jazz - it's just that fish is a staple diet for many people all over the world.

[Cloral]

It seems like oxygen is the best weapon against these bacteria. It oxidizes the hydrogen sulfide making it harmless, and also kills the bacteria itself. In fact, this type of bacteria was dominant across the entire planet before photosynthesis came along and released free oxygen into the atmosphere. That's why its important that we don't let the oceans become oxygen deprived through poor circulation.

[Dann Woolf]

Wait a second.

How can there not be enough oxygen in water?

Water is two parts hydrogen and one part oxygen. If there's no oxygen what you have is a lake of hydrogen.

...damn it, now my brain hurts.

[rock_nog]

No no, oxygen from the atmosphere dissolves into water. Strictly speaking, that's what we're talking about here. The fact that water is one part oxygen is irrelevant, because that oxygen generally remains bonded to the hydrogen atoms. It takes a lot of energy to break that bond, so it doesn't generally happen naturally.